Case 480C Manual Shuttle Shift Issues after rebuilding

I’m first time posting. I bought a Case 480C Backhoe in March and it worked great until about a month ago. While using went to use shuttle shift to go into reverse and it would only grind and jump out of gear. All forward gears and high and low range worked. Tried to adjust linkage but no luck so removed shuttle/Transmission and took to Case Dealer they replaced reverse synchronizer and shift arm we installed it and it won’t go forward just reverse. Sometimes not reverse either. Tried linkage adjustment no luck so I went home and figured so sleep might help. Adjusted linkage some and it worked in forward and reverse great but it went about 500 ft a just locked up and killed the engine. Reverse worked to get back to barn and now it won’t move either forward or reverse. Case dealer is lost on what could be wrong. I’m going to try to manually put it in forward with linkage disconnected tomorrow and see what happens.Any suggestions also what weight of oil should be in transmission Dealer said 90 weight what I drained looks thinner. Thanks